Optical shaft encoders are used to measure the position rotation of a shaft. An optical encoder typically includes a light source, an encoding strip mounted on a disk that moves with the shaft and which modulates the light from the light source, and a photodetector assembly that converts the modulated light signal into electrical signals used for determining the shaft position.
Incremental encoders generate a signal each time the shaft moves through a predetermined angle. If the photodetector assembly includes two photodetectors that are displaced from one another, both the direction and magnitude of the shaft motion can be determined from the signals generated by these photodetectors. A plurality of incremental encoders operating on different “tracks” on the encoding strip can be used to construct an absolute encoder that generates a signal indicative of the position of the shaft relative to a predetermined fixed reference point.
The encoding strips used in both types of encoders can be divided into two broad types. In the first type, the encoding strip consists of alternating opaque and transparent stripes, and the light source and detector array are on opposite sides of the encoding strip. In the second type, the encoding strip consists of alternating non-reflective stripes and reflective stripes, and the light source and detector array are on the same side of the encoding strip.
Encoders of the second type are, in principle, less expensive than encoders of the first type, since the light source and detector array can be combined into a single pre-packaged assembly. One such encoder design is taught in U.S. Pat. No. 5,317,149. This design utilizes two lenses in addition to the light source and the detector array. The first lens is used to generate a light beam that is reflected from the code strip. The second lens images the code strip onto the photodetector array. The cost of the second lens increases the cost of the encoder.
In addition, a single encoder module that will function adequately for a wide range of encoder resolutions is difficult to achieve using this design. The encoding strip is typically constructed by placing the stripe pattern on a disk that rotates with the shaft. The desired resolution sets the pattern of stripes on the encoding strip. The image of a code stripe on the photodiodes is a series of light and dark stripes having a width that must match the size of the active region on the photodiodes. For example, in the case of a two photodiode array used for determining both the direction of motion and the amount of motion, the stripes in the code strip image ideally have a width that is twice the width of the active area on the photodiode. Since the dimensions of the photodiodes are set in the manufacturing process and not easily changed, any mismatch in the dimensions of the stripes must be accommodated by changing the magnification provided by the second lens discussed above. As the resolution changes, the magnification factor may also change. Hence, a single design in which the first and second lenses have fixed focal lengths is often impossible to achieve.
In this type of prior art design, the shape of each stripe in the code strip pattern is a trapezoid. The stripes must be arranged around a disk that rotates with the shaft. Hence, each stripe covers an area defined by two radii on the disk and the angular resolution of the encoder. If the photodetectors have active areas that are rectangles, there is a shape mis-match that reduces the signal to noise ratio in the encoder. Hence, trapezoidal photodetectors are required. This also increases the cost of the encoder, since specialized photodetectors must be utilized.